The present invention relates to a driving method for a liquid crystal device for use as a light valve in a flat panel display, a projection display, a printer, etc.
Various liquid crystal materials have been used in liquid crystal devices, such as nematic liquid crystals, smectic liquid crystals, polymer dispersion-type liquid crystals. Among these, a liquid crystal material classified under a nematic liquid crystal has a long response time of 50 to several hundred msec, and the liquid crystal response is not completed in one frame period (16.7 msec, 60 Hz), so that a picture flow is caused in the case of a motion picture display to result in a poor motion picture quality, thus being unsuitable for motion picture display.
On the other hand, a chiral smectic liquid crystal having a spontaneous polarization has a shorter response time which is nearly one thousandth of that of a nematic liquid crystal, thus allowing a response in one frame period and being considered as suitable for motion picture display.
In recent years, however, it has been clarified that motion picture quality cannot be improved only by a short response time. For example, it has been reported that a continuous lighting-type display device (hold-type display device), such as a liquid crystal device, provides an inferior motion picture quality in principle compared with a pulse lighting-type display device (non-hold-type display device), such as a CRT (cathode ray tube), in xe2x80x9cShingaku Giho EID 96-4 (1996), p. 16xe2x80x9d.
The above report also describes that the motion picture quality of a hold-type display device can be improved by providing a partially non-display period in one frame period which has been conventionally fully used as a display period. The motion picture quality can alo be improved to some extent by adopting a higher display frame frequency of, e.g., 120 Hz (frame period)=8.35 msec), than 60 Hz (16.7 msec).
However, of the above-mentioned display methods, the method of adopting a partially non-display period is accompanied with a difficulty of resulting in an effectively dark display due to a lowering in time-integrated luminance especially in the case where the non-display period is increased.
On the other hand, the method of relying on a higher frame display speed is liable to suffer from a signal transmission delay along panel electrodes and a display irregularity over a panel due to an increased drive frequency.
A principal object of the present invention is to provide a driving method for a liquid crystal device capable of improving the motion picture quality without lowering the luminance or contrast or without causing a signal transmission delay along the electrodes or display irregularity.
According to the present invention, there is provided a driving method for a liquid crystal device of the type comprising: a pair of substrates, a liquid crystal disposed between the substrates so as to form a matrix of pixels arranged in a plurality of rows and a plurality of columns, an electrode matrix for applying voltages to the liquid crystal at respective pixels, and a plurality of active elements each provided to a pixel for supplying a voltage applied to the liquid crystal at the pixel; the driving method comprising driving the liquid crystal device in a succession of frame periods, wherein
each frame period is divided into a plurality (n) of sub-frame periods including at least one (nxe2x88x921) preceding sub-frame period and a final sub-frame period so that said at least one (nxe2x88x921) preceding sub-frame period provides a total period which is shorter than the final sub-frame period,
the active elements along the rows of pixels are sequentially selected row by row at respective selection periods in each sub-frame period, and
the liquid crystal at each pixel is supplied with a voltage in each preceding sub-frame period which is lower than a voltage applied to the liquid crystal at the pixel in the final sub-frame period.
Thus, in the present invention, the total period of the preceding at least one sub-frame period is shortened and an intermediate state between the display and non-display states is displayed during the preceding sub-frame period(s) to improve the motion picture quality while suppressing a lowering in contrast, and the final sub-frame period is made longer to suppress the adverse effect accompanying the signal transmission delay along the panel electrodes.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.